1. Field of the Invention
The present invention relates to telecommunications networks and more particularly to providing high reliability communications to apparatus connected to such networks.
2. Related Art
Public switched telecommunications networks (PSTN) usually comprise local exchanges providing direct service to customers and trunk exchanges which provide interconnection between the local exchanges by way of a trunk network. Typically, a telephone communication involves a customer on one local exchange being connected through the local exchange to a first trunk exchange. A trunk connection is then made to a second trunk exchange to which the destination local exchange is connected. Finally a connection is made through the destination local exchange to the destination customer.
Now in a simple network of the above kind where each local exchange is connected to any one trunk exchange, even if the trunk exchanges are fully interconnected it is possible for a single failure (for example accidental damage to a single cable link between any two of the exchanges in use) to result in substantial call losses.
In practice a more secure network is obtained by connecting each local exchange to two (or more) trunk exchanges so that if a first connection is out of order an alternative route may be attempted.
In early stages flexible routing was introduced in the form of automatic alternative routing (AAR) allowing traffic on overloaded direct routes between the exchanges to be diverted via alternative pre-assigned routes. In AAR the alternative routes from a given source node to a given destination node of the network are predetermined in a fixed ranking so that for each call blocked on the direct route the control equipment always offers the first choice alternative route. If the first choice alternative route is not available then the second choice alternative route is tried until all of the alternative routes are tested.
In a further development of AAR such as that disclosed in U.S. Pat. No. 3,394,231, longer routes involving a plurality of trunk exchanges may be offered as alternative routes to take advantage of faster switching speeds of modern telecommunications equipment.
In a still further development of AAR disclosed in European patent number 229494, a form of sticky-random alternative routing is disclosed in which calls between a first node and a destination node are offered by a preferred route and if the preferred route is not available calls are directed to a current nominated alternative route and each call failing on the first preferred route continues to be directed to the same alternative route until the nominated alternative route also fails at which point a replacement nomination for the current alternative route is made.
Such systems are useful when a common network management system and/or a common channel signalling system such as CCITT Signalling System 7 (c7) are in use. However, where the PSTN is using a different signalling system to that of connected equipment, for example when providing interconnection between customers private branch exchanges (PBX), the signalling system in use between PBXs is likely to be of the kind known as DPNSS. Thus, simply connecting a PBX to a plurality of local exchanges may result in end-to-end call clearing by the PBX signalling without the network recognising that a failure has occurred. In practice DPNSS is likely to send a CRM (congestion) message from the terminating local exchange to the originating PBX the message being imbedded in a C7 (NUP) signalling message.
According to a first aspect of the present invention there is provided a method of routing calls in a communications network in which a destination has a single network number and at least two network addresses, each network address representing a connection to a respective destination telephone exchange, the method comprising, on receipt of a call for the destination number, analysing the network number to select which of the at least two network addresses the call is to be directed to such that successive calls are directed to available ones of the network addresses in a sequential manner, selecting a route through the network to the selected network address and monitoring network signals to determine the success or failure of the call, and, in the event of failure of the call, determining whether calls to the currently selected network address have failed on a predetermined number of consecutive call attempts and, if so, barring subsequent calls to the selected network address until such time as calls directed to another of the at least two network addresses have failed a predetermined number of times.
According to a second aspect of the present invention a telecommunications network comprising a multiplicity of local exchanges interconnected by a plurality of trunk exchanges such that calls arising on any network address designating a customer connection to one of the local exchanges may be connected to any other network address in the communications system, the system translating digits defining a telephone number received from any network address into a destination network address to permit the establishment of a communications call between a calling network point and a destination network point characterised in that at least one telephone number represents a plurality of network addresses at least some of which on a first of said local exchanges and at least some others of which are on a different one of said local exchanges, each of said network addresses representing a route to a common customer communications equipment destination, control means of the network being responsive to signals received indicating a call for said customer equipment to analyse the number to select which of the network addresses the call is to be directed to such that successive calls are directed to network addresses representing different available routes in a sequential manner, the control means causing a route to be set up through the network to the selected network address and monitoring network signals to determine success or failure of the call and in the event of failure of the call determining from data relating to the selected network address if a predetermined number of consecutive call attempts to the selected network address have failed and if so barring subsequent calls to the selected network address until such time as calls directed to another of the local exchanges has failed a predetermined number of times.
Preferably the communications network is in the form of an intelligent network comprising a service control point and service switching point, the service switching points being triggered to apply to the service control point for routing information to handle calls to designated network telephone numbers representing a multiply connected PBX.